1. Field of the Invention
The invention relates to synchronous networks and more particularly to switching a primary reference clock source in a synchronous network.
2. Description of the Related Art
The characteristics of a primary reference clock (PRC) are established in the ITU-T Recommendation G.811 and in ANSI Level 1. Simplified, it deals with a reference clock supply, which makes available a clock or synchronization signal having a long term deviation as against UTC (Universal Time Coordinated) of at the most 1xc3x9710 exp(xe2x88x9211). The generation of this time signal can occur completely autonomously. However, it can also occur with dependence on sources of signals derived from the UTC directly.
A device which generates a PRC clock contains, as a rule, a source of signals and a signal processor, as well as an output interface. With an autonomous PRC, the source of signals is a highly stable reference oscillator, which is implemented as a cesium oscillator, maser oscillator, or rubidium oscillator. For non-autonomous generation of the time signal this reference oscillator itself is synchronized to an external signal. This can be derived from a GPS (Global Positioning System) receiver or a receiver of time signals of a different kind, e.g. DCF77, LORAN-C, GLONASS. The output interface makes available a clock or data signal which is derived directly from the signal of the reference oscillator. For output signals there is a choice of 2048 kHz, 2048 kbit/s, 1544 kbit/s, 1 MHz, 5 MHz or 10 MHz according to G.811, with other signals conceivable there as well. For reasons of availability the reference oscillator and the output interface are often provided redundantly. The signal processing then selects, dependent on many criteria, one of the reference oscillator signals or else an average value signal for the output interface. The configuration and alarm indicator of the PRC occurs via a service terminal. The service terminal, connected via a data line, can also be located elsewhere.
In order to increase the availability of PRCs, these are, as a rule, extant doubly in the network. In order to provide better protection against external influences, it is recommended to position the PRCs in geographically diverse locations (replacement switching of geographically redundant PRCs). This so-called geographic redundancy guarantees the highest availability. If one PRC fails, the other PRC remaining in the network must take over the central clock generation. One PRC functions as master (PRC1) in the network, while the other one functions as slave (PRC2). For the slave itself to become master if the master fails, it must obtain the information about the failure of the master. To that end, customarily, a separate data channel is used in the SDH network (Synchronous Digital Hierarchy), for example a 64 kbit/s data line; compare FIG. 3. On this data channel the state information of the master is transferred to the slave, and vice versa. Such a solution is especially expensive and cost intensive, because for the coupling and decoupling of the data signal into the STM signal (Synchronous Transport Multiplex) separate equipment is necessary. Besides, this channel must be installed likewise doubly or redundantly in order to make available an alternate routing in case of outage.
The present invention provides automatic switching of primary clock reference sources in case of reference malfunction in a SDH network. One clock source is treated as a master and another clock source is treated as a slave. Switching between the master and slave is controlled using a Synchronization Status Message which is communicated over the network.